Fungal and bacterial pathogens can lower yields, reduce quality, negatively affect the aesthetic and economic value and even ultimately destroy plants, crops, pre-harvest fruits, trees, vegetables and grasses.
A broad range of organic molecules have been found to have fungicidal and bactericidal properties and are effectively used for plant disease control. However, many of the currently used pesticides pose a high risk to human health and the environment and are not biodegradable. Since the establishment of the Environmental Protection Act in 1972 there has been an increased concern over the use of toxic chemicals for plant disease control and the dangerous residual potential these toxic products represent. The United States Congress disclosed its concerns with those products with the passage of the Food Quality Protection Act in August, 1996 which requires the U.S. EPA to reassess each existing pesticide by 2006. Because of toxicity concerns, to reduce residues on crops, fruits and vegetables, the application of pesticides shortly before harvest must usually be avoided. Additionally, because of concerns regarding the health of workers, entry into fields or greenhouses shortly after pesticide application is usually prohibited.
Therefore, there is a real need to provide more biocompatible fungicides and bactericides which are, by definition, safe in the environment, non-toxic to humans and animals, and which are rapidly biodegradable.
Because of their safety and efficacy, preservatives have long been used to prevent growth of stray bacteria, fungi and other pathogens in cosmetics, household products, foods and beverages. These materials are thereby effective in preserving products against spoilage by fungal and bacterial contamination and do not present a health or safety hazard when regularly contacted or ingested in minor concentrations. It has been demonstrated that preservatives can prevent storage rot of post-harvest fruits such as bananas, citrus fruit, potatoes and yams caused by fungi and bacteria (Al-Zaemey et al. “Studies on the effect of fruit coating polymers and organic acids on growth of Colletotrichum musae in vitro and on postharvest control of anthracnose of bananas”. Mycol. Res 97: 1463-1468, 1993; Kitagawa, H., Kawada, K. “Effect of sorbic acid and potassium sorbate on the control of sour rot of citrus fruits.” Proc. Ann. Meet. Fla. State Hortic. Soc. s. Vol 97: 133-135, 1985; Hervieux, V., et al. “Effect of organic and inorganic salts on the development of Helminthosporium solani, the causal agent of potato silver scurf.” Plant Dis. 86: 1014-1018, 2002; Yaganza, E. S. et al. “Postharvest application of organic and inorganic salts for the control of potato tuber soft rot.” Phytopathology 91: S 198, 2001.)
While it has been suggested that some preservatives may be environmentally safe and effective in controlling plant diseases caused by fungi, the parameters for the usefulness thereof have not been thoroughly explored. U.S. Pat. No. 5,057,326 states that sodium propionate, sodium sorbate and sodium metabisulfite are each known to be fungicidal when used alone at high concentrations. Data are provided which show that, when used alone in fungicide formulations at concentrations of 0.03% to 0.23%, potassium sorbate, sodium propionate or sodium metabisulfite are only marginally effective (25 to 40% reduction) in controlling powdery mildew on spring barley. However, the patent describes the synergistic use of combinations of these water soluble salts to control fungi on growing plants and post-harvest fruits. Thus, data is provided showing that when sodium metabisulfite is mixed with either potassium sorbate or sodium propionate the efficacy is greatly improved (90 to 100% reduction). While sodium metabisulfite is a slightly acidic salt (pH alone between 4.5 and 5.0) combinations with the propionate and sorbate salts would be considerably more basic. The criticality of pH on activity is not suggested. Further, the addition of sodium metabisulfite to a plant fungicide has three potential undesirable side-effects. First, sodium metabisulfite emits sulfur dioxide at acidic pHs creating a potentially toxic atmosphere for humans. Thus it would be undesirable to enter fields or greenhouses immediately after treatment with products containing sodium metabisulfite. Secondly, sulfur dioxide emissions are environmentally undesirable in that they can contribute to acid rain development. Thirdly, residues of sodium sulfite can remain on treated plants. Many people are allergic to sodium sulfite in foods. Therefore, metabisulfite-containing fungicides cannot be applied to fruits and vegetables immediately before harvest.
Similarly, U.S. Pat. No. 4,851,223 discloses the use of preservatives such as sodium metabisulfite, sodium propionate, etc. in a mixture with a coating agent di-1-p-menthene and, optionally, a nonyl phenol ethylene oxide surfactant to control fungi and bacteria on living ornamentals and crop plants. Once again the criticality of pH is not mentioned and the same potential negative safety and environmental effects from metabisulfite exist with this invention as with those of U.S. Pat. No. 5,057,326.
U.S. Pat. No. 5,750,402 discloses the use of potassium sorbate and sodium benzoate as part of a formulation containing the biocide mix of methylchloroisothiazolinone and methylisothiazolinone used to prevent microbial growth in plant tissue culture and plant seed growth media. However, the criticality of pH is not mentioned, indeed both potassium sorbate and sodium benzoate are alkaline salts. As a result, the amounts of potassium sorbate and sodium benzoate needed are high.
When used alone, preservatives would not be expected to be useful in controlling diseases on plants, crops, trees, pre-harvest fruits, flowers, vegetables or grasses. First of all, preservatives are generally growth inhibitory but they are often not very effective in killing fungi or bacteria. Secondly, preservatives are not known to have any plant systemic activity, which is usually associated with effective plant fungicides and bactericides. Thirdly, preservatives do not spread well when applied to plant tissue, leaving large untreated areas where fungi and bacteria can survive. Fourthly, when used in cosmetics, foods and beverages the levels of fungi and bacteria are limited and there is an extended period of time available, sometimes several weeks, for the preservatives to kill the pathogens. In fact when used in preserving cosmetics and foods, it is often not necessary to kill all the bacteria and fungi present. It is often only needed to prevent their further growth and proliferation. On the other hand, in plants there are often high levels of fungi and bacteria that have to be eradicated and the period of time available for activity may be limited due to rain and other prevalent conditions. Optimum conditions for the efficacious use of preservatives on live plants have not been previously reported. Therefore, preservatives are not generally used to provide control of diseases on or in living plants